Very Airtight Construction

Fixing the Holes Where the Air Gets In, Journal of Light Construction, January 2004, is a detailed and well illustrated presentation of one contractor's techniques for minimizing air leakage through light wood frame constructed building envelopes. Techniques include:

• Overall building leakage rates are measured using a calibrated blower door. This contractor aims for passive rates of exchange of 1/10 air change per hour. Lessons learned from diagnosing leaky details through such testing are applied to future projects.
• Foundation sill plates are set on EPDM rubber gaskets. Rim joists are sealed to the sill with adhesive or sealant. The top edge of the rim joist is also sealed to the underside of the subfloor with construction adhesive.
• Neoprene gaskets are placed between the bottom of exterior wall frames and the subfloor.
• Sealing tape is applied to all horizontal exterior sheathing joints and other joints not backed up by solid framing.
• Holes in framing for plumbing or wiring are sealed with expanding foam caulk.
• The gap between door and window frames and rough opening framing is sealed with EPDM gaskets. Gaps too narrow for gaskets are sealed with silicone caulk.
• Gaps between firestopping and flues or chimneys are sealed with high-temperature silicone caulk.
• Ceiling end bays that intersect with ventilation channels at the roof eave are sealed with custom-cut rigid foam panels.
• Tops of interior partitions are sealed with sprayed expanding foam to seal the wall/ceiling framing juncture.
• Walls and ceilings are insulated with dry-blown cellulose. Where possible, insulation values are upgraded with 3/4- or 1-inch foil faced rigid foam insulation applied on the interior side of exterior walls (also with seams taped). Hard to reach areas are insulated with two-component polyurethane foam.
• Vapor barrier membrane is either Hanes Industry's 100% polypropylene or Certainteed's proprietary variable perm rate "MemBrain" product.
• Mechanical exhaust systems are sized for a minimum of 15 cubic feet of air change per hour per building occupant. Bathrooms fans can be manually or automatically boosted for high-humidity conditions. Heat recovery ventilation systems are also sometimes installed to increase energy efficiency.

While the techniques illustrated in this article may not be suitable for every builder or every project, this article illustrates field-tested, effective techniques for achieving low-leakage rate residential building envelopes.